When Good Scientists Reach Different Conclusions
In the previous article, we explored one of the biggest misconceptions surrounding skeletal muscle. Rather than asking whether muscle mass matters, we concluded that a much more useful question is which characteristics of skeletal muscle best predict long-term health, functional capacity, and longevity. As it turns out, muscle quantity, muscle quality, strength, power, metabolic function, and cardiorespiratory fitness each provide unique insights into the same remarkably complex physiological system.
That conclusion naturally raises another question: If healthy skeletal muscle is truly multidimensional, why does the scientific literature sometimes appear so divided?
Over the past several years, muscle-centric medicine has become one of the more widely discussed topics in healthy aging, longevity, and exercise science. Some researchers have argued that skeletal muscle should be viewed as a primary driver of metabolic health and healthy aging. Others have suggested that the field has placed too much emphasis on muscle mass itself, pointing instead toward more traditional measures of muscular strength, gait speed, muscle power, cardiorespiratory fitness, and visceral adiposity as stronger predictors of clinically meaningful outcomes. At first glance, those positions seem fundamentally incompatible.
After spending considerable time working through the literature, however, I’m not convinced the disagreement is as large as it initially appears.
On the contrary, in many respects, I think the debate has less to do with whether skeletal muscle matters and much more to do with how skeletal muscle is being measured.
That distinction is subtle, but it has important implications for anyone attempting to interpret the current evidence.
One of the most consistent themes emerging in recent literature is that different assessment methods capture distinct characteristics of skeletal muscle. Dual-energy X-ray absorptiometry (DXA), for example, remains one of the most widely used methods for estimating lean tissue in both research and clinical practice because it is relatively accessible, cost-effective, and practical.
However, as Bennett and colleagues (2025) recently emphasized, DXA does not directly measure skeletal muscle. Instead, it estimates appendicular lean soft tissue, a compartment that includes skeletal muscle alongside water, connective tissue, and other non-fat, non-bone tissues. As a result, differences in terminology and interpretation have contributed to considerable confusion throughout both the scientific literature and clinical practice (Bennett et al., 2025).
That methodological limitation becomes especially important when interpreting recent criticisms of muscle-centric medicine.
One widely discussed critique argued that measures such as grip strength, gait speed, muscle power, and cardiorespiratory fitness consistently outperform DXA-derived lean mass when predicting disability, mobility limitation, hospitalization, and mortality in older adults. Viewed in isolation, those findings are both accurate and important. Functional measures often do provide stronger prognostic information than DXA-derived lean mass in many clinical populations.
The difficulty arises when that observation is extended to a much broader conclusion—that measuring muscle is somehow “measuring the wrong thing.”
Interestingly, that is not the conclusion reached by many of the investigators whose work is frequently cited within this debate. Instead, several authors have suggested that the apparent shortcomings of DXA may reflect limitations of the measurement itself rather than evidence that skeletal muscle is physiologically unimportant. In fact, both the Sarcopenia Definitions and Outcomes Consortium (SDOC) analyses and more recent methodological reviews acknowledge that DXA-derived lean mass represents an indirect estimate of skeletal muscle and that newer techniques, including D3-creatine dilution, may provide a more accurate representation of true contractile muscle tissue.
That distinction fundamentally changes how we interpret the debate.
Rather than asking whether muscle mass matters, the more appropriate question may be whether we have been measuring muscle mass accurately enough to fully appreciate its relationship with health, function, and disease risk.
As we’ll see throughout the remainder of this article, the answer to that question appears considerably more nuanced than many of the headlines would suggest.
What the SDOC Papers Actually Tell Us
One of the papers that has received considerable attention in recent discussions surrounding muscle-centric medicine comes from the SDOC. Drawing on data from more than 18,000 older adults across multiple prospective cohorts, the investigators sought to determine which skeletal muscle characteristics were most strongly associated with clinically meaningful outcomes, including mobility limitation, falls, fractures, hospitalization, disability, and mortality. Rather than assuming muscle mass alone defined sarcopenia, the consortium systematically evaluated multiple candidate measures—including DXA-derived appendicular lean mass, grip strength, and gait speed—to determine which best predicted real-world function and health outcomes (Kirk et al., 2024).
The findings were both important and, in many respects, practice-changing.
Across the consortium analyses and numerous subsequent investigations, measures of physical function—particularly grip strength and gait speed—consistently demonstrated stronger associations with disability, functional decline, and mortality than DXA-derived appendicular lean mass alone (Bohannon, 2019; Lee, 2019; Xiong et al., 2023; Mayhew et al., 2023). These observations helped shift the field away from defining sarcopenia exclusively by muscle quantity and toward recognizing the importance of muscle function when evaluating older adults (Kirk et al., 2024).
Truthfully, I think this represents an important advancement for both clinicians and exercise professionals.
After all, our clients rarely care about the number of kilograms of lean tissue they possess. They care about whether they can continue hiking with their grandchildren, recover from illness, climb stairs without assistance, or remain independent as they age. Measures such as grip strength and gait speed capture much of that reality because they integrate multiple physiological systems simultaneously. Rather than reflecting skeletal muscle alone, they also incorporate neuromuscular coordination, balance, cardiovascular function, cognitive processing, and overall physiological reserve (Bohannon, 2019; Forman et al., 2017; Manzo et al., 2022; Takino et al., 2026).
Where I think the conversation becomes considerably more nuanced, however, is in how these findings have sometimes been interpreted.
In recent years, some commentators have suggested that because grip strength and gait speed outperform DXA-derived lean mass, measuring muscle mass is therefore “measuring the wrong thing.” While that interpretation certainly makes for an engaging headline, it goes beyond what the available evidence actually demonstrates.
One of the most important observations emerging from the broader literature is that DXA does not directly measure skeletal muscle. Instead, it estimates appendicular lean soft tissue—a composite measure that includes skeletal muscle alongside connective tissue, water, and other non-fat, non-bone tissues (Bennett et al., 2025; Kim et al., 2004). As Bennett and colleagues (2025) argue, inconsistent terminology surrounding DXA-derived measurements has contributed to confusion throughout both the scientific literature and clinical practice. Consequently, a poor association between DXA-derived lean mass and clinical outcomes does not necessarily imply that skeletal muscle itself lacks physiological importance. Rather, it raises the possibility that the surrogate measure being used may not perfectly reflect the biological tissue researchers are attempting to study.
Interestingly, newer methodologies appear to support that possibility.
The D₃-creatine dilution method, for example, provides a more direct estimate of skeletal muscle mass than DXA and has demonstrated stronger relationships with incident mobility disability, fracture risk, and age-related functional decline (Zanker et al., 2020; Cawthon et al., 2022; Wimer et al., 2023). Likewise, advanced imaging techniques such as computed tomography (CT) and magnetic resonance imaging (MRI) allow investigators to evaluate not only muscle quantity but also muscle quality, radiodensity, and intramuscular fat infiltration—characteristics increasingly recognized as important determinants of metabolic health and physical function (Rollins et al., 2019; Zaffina et al., 2021; Ha et al., 2021; Pinel et al., 2021).
From that perspective, the apparent disagreement within the literature begins to look much less like a contradiction.
Perhaps the SDOC investigators were not demonstrating that skeletal muscle is unimportant. Perhaps they were demonstrating that how we measure skeletal muscle profoundly influences the conclusions we draw. When viewed through that lens, the debate shifts away from whether muscle matters and toward a far more interesting scientific question: Which characteristics of skeletal muscle—and which methods of measuring them—best predict long-term health, function, and longevity?
Looking Beyond Muscle Mass: Why Muscle Quality Matters
If there’s one theme that consistently emerged while reviewing the literature, it’s that skeletal muscle cannot be evaluated solely by asking how much of it someone has. Increasingly, researchers recognize that the quality of skeletalmuscle may be just as important as its quantity, particularly for predicting metabolic health, physical function, and healthy aging (Pinel et al., 2021; Marcus et al., 2012).
At first glance, that distinction may seem subtle, but it fundamentally changes how we think about muscle.
For decades, body composition assessments have largely focused on estimating the amount of lean tissue an individual possesses. While that information certainly has value, two people with nearly identical amounts of skeletal muscle may have dramatically different physiological profiles. One individual may possess well-vascularized muscle with high mitochondrial density, healthy contractile tissue, and relatively little fat infiltration. Another may have similar muscle size but substantially greater intramuscular adipose tissue, reduced capillary density, impaired mitochondrial function, and diminished force-producing capacity (Gavin et al., 2004; Mogensen et al., 2007; Pinel et al., 2021).
From the outside, those two individuals might appear remarkably similar. Physiologically, however, they are anything but.
This concept of muscle quality has become an increasingly important area of investigation because it helps explain why muscle size alone does not always predict function. In one study, older adults demonstrated muscle volumes similar to those of their younger counterparts, yet significantly greater intramuscular fat infiltration and lower mechanical muscle properties, both of which were associated with reduced strength (Pinel et al., 2021). Likewise, Marcus and colleagues (2012) reported that intramuscular adipose tissue explained more variation in mobility than lean tissue itself, suggesting that what exists within the muscle may sometimes be more informative than the amount of muscle present.
In many respects, this should sound familiar.
Cardiology has long recognized that two individuals with hearts of similar size may have vastly different cardiovascular health depending on factors such as myocardial function, vascular integrity, and cardiac output. Likewise, possessing larger skeletal muscles does not automatically imply that those muscles are functioning optimally. Healthy muscle is characterized not only by its size but also by its ability to contract efficiently, produce force rapidly, utilize oxygen effectively, regulate glucose, and respond appropriately to metabolic demands (Hoffmann & Weigert, 2017; Blackwood et al., 2022).
This distinction also helps reconcile much of the apparent disagreement discussed earlier in the article.
If traditional body composition techniques primarily estimate the amount of lean tissue, while functional tests capture the integrated performance of muscle, the nervous system, cardiovascular function, and movement efficiency, it becomes far less surprising that these measurements sometimes yield different conclusions. They are not necessarily measuring competing concepts. Rather, they are describing different characteristics of the same physiological system (Bennett et al., 2025; Kirk et al., 2024).
For personal trainers, I think this perspective is incredibly liberating.
It reminds us that successful training programs should never be judged exclusively by whether a client adds a few pounds of lean mass. Instead, we should also ask whether they are producing more force, moving with greater confidence, recovering more effectively, improving work capacity, and demonstrating better movement quality than they did several months ago. Those changes often reflect meaningful improvements in muscle health, even before large changes in body composition become apparent (Liu & Latham, 2009; Cadore, 2014).
Perhaps most importantly, this evolving understanding prepares us for the next question.
If healthy skeletal muscle is defined by far more than its size, then why does improving muscle quality produce such widespread benefits throughout the body? The answer lies in one of the most significant paradigm shifts in modern exercise physiology: the recognition that skeletal muscle is far more than a mechanical tissue. It is also one of the body’s most important metabolic and endocrine organs (Hoffmann & Weigert, 2017).
Healthy Muscle Is Working Even When You’re Not
One unintended consequence of the recent debate over muscle mass is that it can narrow our view of what skeletal muscle actually does.
If we think of muscle primarily as a tissue responsible for producing force, then it makes perfect sense to judge it by how much strength someone can generate or how quickly they can walk. Those outcomes are certainly important. They represent many of the real-world abilities that allow people to remain active and independent throughout life.
But skeletal muscle does considerably more than produce movement.
Even while we are sitting, sleeping, or reading this article, skeletal muscle remains one of the body’s largest and most metabolically active organs. It serves as the primary site of insulin-mediated glucose disposal, stores substantial amounts of glycogen, continuously turns over protein, contributes to whole-body energy expenditure, and communicates with numerous organs through an intricate network of endocrine and metabolic signaling pathways (Hoffmann & Weigert, 2017; Handschin et al., 2007; Smith et al., 2018).
That perspective helps explain why exercise yields benefits that extend far beyond the gym.
For example, resistance training has repeatedly been shown to improve insulin sensitivity, even in the absence of dramatic changes in body weight. At the cellular level, muscle contractions stimulate increases in GLUT4 transporters, insulin receptors, glycogen synthase activity, and multiple proteins involved in insulin signaling, ultimately allowing skeletal muscle to dispose of glucose more efficiently (Holten et al., 2004). In many individuals with insulin resistance or Type 2 diabetes, these adaptations occur well before meaningful changes in body composition become apparent.
This is one of the reasons exercise physiology can be so fascinating.
Clients often assume that because the scale hasn’t changed, “nothing is happening.” In reality, profound physiological remodeling may already be underway. Mitochondria become more efficient. Capillary networks expand. Muscle fibers improve their ability to oxidize fat and utilize glucose. Insulin signaling becomes more effective. Inflammatory signaling decreases while anabolic pathways become more responsive (Mogensen et al., 2007; Gavin et al., 2004; Assar et al., 2022).
Increasingly, researchers have begun describing skeletal muscle as an endocrine organ as well.
During and after exercise, contracting muscles release numerous signaling molecules known as myokines. Rather than acting exclusively within the muscle itself, these molecules influence tissues throughout the body, helping regulate glucose metabolism, lipid metabolism, inflammation, angiogenesis, cognitive function, immune activity, and even communication between skeletal muscle and the pancreas (Hoffmann & Weigert, 2017; Handschin et al., 2007). In other words, every bout of exercise initiates a coordinated physiological response that extends well beyond the muscles performing the work.
From that perspective, the conversation surrounding muscle-centric medicine begins to look very different.
The question is no longer whether skeletal muscle predicts health because it helps someone generate force. The question is whether healthy skeletal muscle directly contributes to the regulation of many physiological systems associated with healthy aging. Increasingly, the evidence suggests that it does (Blackwood et al., 2022; Hoffmann & Weigert, 2017; Strasser et al., 2018).
Interestingly, this also helps explain why so many seemingly different measurements predict health outcomes.
Grip strength reflects the functional expression of the neuromuscular system. Gait speed integrates multiple physiological systems simultaneously. Muscle quality reflects the structural integrity of skeletal muscle. Cardiorespiratory fitness captures the body’s ability to transport and utilize oxygen. Although each measurement approaches the question from a different angle, they are all describing interconnected components of the same biological system rather than competing explanations of human health (Forman et al., 2017; Kirk et al., 2024).
Perhaps that is why the current literature feels far less contradictory once we step back and look at the broader picture. Rather than arguing over which single variable matters most, researchers are gradually revealing just how interconnected healthy skeletal muscle truly is.
What This Means for Personal Trainers
So, where does all of this leave the personal trainer?
Perhaps with a much clearer understanding of what our role has become.
Clients are no longer walking into our facilities having read only textbooks or position stands. Increasingly, they arrive after listening to podcasts, scrolling through social media, and hearing experts debate whether muscle mass matters, whether strength matters more, or whether body composition has been overemphasized altogether. It’s understandable that many leave feeling confused.
Our responsibility isn’t to choose sides. Rather, our responsibility is to provide context.
One of the biggest lessons from the literature reviewed throughout this article is that physiology rarely operates in absolutes. Muscle quantity matters. Muscle quality matters. Strength matters. Power matters. Cardiorespiratory fitness matters. Body fat distribution matters. Each contributes unique information about health, function, and disease risk, and each interacts with the others through remarkably complex physiological pathways (Bennett et al., 2025; Forman et al., 2017; Kirk et al., 2024).
Rather than competing explanations, they are complementary pieces of the same biological system. That perspective changes the way we coach.
Instead of celebrating only the number on the scale or a body composition report, we begin asking different questions.
- Is our client becoming stronger?
- Are they moving more confidently?
- Can they tolerate greater training volumes?
- Has climbing stairs become easier?
- Are they recovering more quickly between sessions?
- Do they feel more physically capable than they did six months ago?
Those questions often tell us considerably more about meaningful progress than any single laboratory measurement ever could.
At the same time, none of this should be interpreted as an argument against measuring body composition. Assessment remains an essential part of professional practice. The key is understanding what each assessment actually tells us. DXA, D3-creatine dilution, grip strength, gait speed, muscle power, cardiorespiratory fitness, and anthropometric measurements all contribute valuable information. Problems arise only when we expect one measurement to explain the entirety of human health.
Perhaps that’s the biggest takeaway from this discussion.
The recent debate surrounding muscle-centric medicine has generated far more light than heat, encouraging researchers, clinicians, and exercise professionals to think more carefully about what skeletal muscle actually represents. Rather than asking whether muscle mass alone predicts health, the literature increasingly encourages us to appreciate skeletal muscle as a dynamic tissue whose size, quality, metabolic activity, strength, and functional performance collectively influence health across the lifespan (Hoffmann & Weigert, 2017; Kirk et al., 2024).
Ironically, once we adopt that broader perspective, many of the apparent disagreements begin to disappear.
Hence, science isn’t telling us that muscle mass no longer matters. On the contrary, it’s reminding us that muscle has always been far more than mass alone.
Conclusion
At first glance, the ongoing debate surrounding skeletal muscle appears surprisingly polarized. Depending on which article or podcast someone encounters, muscle mass is either portrayed as the central predictor of health or dismissed in favor of strength, power, gait speed, or cardiorespiratory fitness.
The current evidence supports neither extreme.
Instead, it paints a far richer picture of skeletal muscle as both a mechanical tissue and one of the body’s most influential metabolic organs. It regulates movement, glucose metabolism, inflammation, energy homeostasis, and functional independence while continually adapting to the demands we place upon it (Hoffmann & Weigert, 2017; Bennett et al., 2025; Kirk et al., 2024).
For personal trainers, that’s encouraging news. It means our profession has never been about helping clients chase a single number. It has always been about helping people build bodies that are stronger, healthier, more resilient, and better prepared for the demands of everyday life.
Perhaps that’s why the biggest debate in exercise science isn’t really about muscle mass at all.
It’s about recognizing that healthy skeletal muscle cannot be reduced to a single measurement.
And once we begin viewing muscle through that broader lens, many of today’s scientific debates become considerably less controversial—and considerably more useful for the clients we serve.
Practical Takeaways for Personal Trainers
As you apply these concepts in practice, keep four principles in mind:
- Continue using body composition assessments, but recognize that they represent one piece of a much larger physiological picture.
- Measure function alongside structure whenever possible. Strength, power, work capacity, movement quality, and cardiorespiratory fitness provide valuable context that body composition alone cannot capture.
- Help clients appreciate the invisible adaptations occurring beneath the surface. Improvements in insulin sensitivity, metabolic health, muscular function, and resilience often precede visible changes in appearance.
- Resist false dichotomies. Exercise science continues to evolve, but the strongest professionals rarely search for a single “best” metric. Instead, they learn how multiple physiological systems work together to support long-term health and performance.
As our understanding of skeletal muscle continues to expand, one conclusion becomes increasingly difficult to ignore:
Healthy muscle is about far more than muscle mass alone.
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